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van der waals Forces

van der waals forces are forces which hold molecules of the same substance together. They do not depend on the bond making the particular molecules; they make it possible for gaseous compounds like ammonia, nitrogen and carbon(IV) oxide to exist in liquid or solid state.

The presence of these forces between the molecules of a substance can be explained thus: under suitable temperatures and pressures for molecules to be crowded close together, the atomic nuclei of one molecule attract the electron cloud of another appreciably, causing the two to come closer. As they come together, they experience repulsive forces between their electrons, which tend to pull them apart.

However, the forces of attraction and repulsion are balanced, resulting in the formation of a crystal - these forces (i.e., attraction and repulsion between molecules) are called van der waals forces (named after the discoverer). These forces are rather weak, hence, the crystals have low melting points. Compounds where van der waals forces apply strongly are large (containing more electrons), non polar molecular solids, such as naphthalene crystal, iodine crystals and graphite.